Molecular recognition by calix[4]arene-modified gold nanoparticles in aqueous solution

Article abstract of DOI:10.1002/anie.200462909

(Chemical Equation Presented) Extremely stable, water-soluble gold nanoparticles 3 capable of specific molecular recognition of pyridinium ions in aqueous systems are prepared. The particles are stabilized and solubilized by thioalkylated oligoethylene gl

Full text of DOI:10.1002/anie.200462909

Angewandte
Chemie
[
20]
Nanoparticles
MPCs can enhance their affinity to guest molecules. By
themselves, calixarenes are completely insoluble in water,
which significantly limits their potential applications as
molecular recognition units. The sulfanylalkyl oligo(ethylene
glycol) ligands in the system described herein act therefore
not only as stabilizers for the MPCs but also as solubilizers for
the attached calix[4]arene units. It is thus now possible to
demonstrate molecular recognition by an unmodified calix-
arene cavity in water. The particles prepared have been
characterized by transmission electron microscopy (TEM)
and by UV/Vis, and NMR spectroscopy. Specific recognition
of immobilized cationic pyridinium moieties by the calixar-
ene-modified MPCs in aqueous solution has been demon-
strated by a simple color test and by atomic force microscopy
(AFM).
Molecular Recognition by Calix[4]arene-
Modified Gold Nanoparticles in Aqueous
Solution**
T. Robert Tshikhudo, Domenico Demuru,
Zhenxin Wang, Mathias Brust,* Andrea Secchi,
Arturo Arduini, and Andrea Pochini*
The concept of exploiting metal clusters for nanotechnology
applications dates back to the development of the original
[
1]
Au55 cluster by Schmid et al. in 1981. Over the past decade,
so-called monolayer protected clusters (MPCs) of gold and, to
a lesser extent, silver have been studied extensively owing to
their extreme stability and the plethora of tunable properties
that are controlled by the particle size and by the ligand
The preparation of the calixarene-modified nanoparticles
is schematically illustrated in Figure 1. Citrate-stabilized gold
[
2–8]
chemistry.
The stability is usually achieved by the use of
thiolate ligands, which form a protective shell around the
particles to which they are attached by the strong AuÀS
interaction. Typical sizes of MPCs range from 1 to approx-
imately 40 nm depending on the preparation method used.
Since most preparations lead to materials that are insoluble in
water a comparatively small number of studies have been
[
9–18]
carried out in aqueous systems.
In particular, in view of
future bioanalytical applications it is, however, desirable to
develop new MPC systems that are not only stable and
soluble in water, but also capable of molecular recognition in
aqueous systems. Very stable, yet chemically versatile water-
soluble MPCs are obtained when a sulfanylalkyl oligo(ethy-
[
16]
lene glycol) is used as a stabilizing ligand. Herein we report
the preparation and chemical properties of 14-nm gold MPCs,
which are stabilized in this way and, in addition, carry in their
ligand shell calix[4]arene moieties, which confer their specific
molecular recognition properties to the particles. Calixarenes
are host molecules which can bind cationic guests strongly and
with high specificity depending on the size of the calixarene
Figure 1. Reaction scheme illustrating the one-step stabilization and
functionalization of gold nanoparticles with 1 and 2 carried out in a
THF/water mixture.
[
19,20]
cavity.
Some of us have recently shown that, in non-
aqueous systems, the incorporation of calixarenes in 1.5–4-nm
hydrosols were treated with a 2:1 mixture of the stabilizing
ligand (1-sulfanylundec-11-yl) tetraethylene glycol (1) and
2
5,27-bis(11-thio-1-oxyundecan)-26,28-dihydroxycalix[4]ar-
[
*] T. R. Tshikhudo, Dr. Z. Wang, Dr. M. Brust
Centre for Nanoscale Science
Department of Chemistry
University of Liverpool
Liverpool L697ZD (UK)
ene (2) in a water/THF solvent system to directly give the
modified particles 3, which were isolated from excess ligand
material by centrifugation. Given the conformation of the
relatively rigid 2 it is reasonable to assume that both thiol
functionalities of 2 bind to the surface of the same gold
particle. Otherwise, cross-linking of particles by bisthiol
Fax: (+44)151-794-3588
E-mail: m.brust@liv.ac.uk
[
21]
D. Demuru, Dr. A. Secchi, Prof. A. Arduini, Prof. A. Pochini
Dipartimento di Chimica Organica ed Industriale
dell’Universitꢀ
Parco Area delle Scienze 17/a, 43100 Parma (Italy)
Fax: (+39)521-905472
bridges and precipitation of aggregates could be expected,
but this has not been observed in this case. On the contrary,
the particles are extremely stable and can be centrifuged,
dried, and re-suspended in aqueous solution several times
without loss of material. The ruby red suspensions are very
robust against non-specific aggregation. For example, unlike
most other hydrosols, the particles do not aggregate even in a
E-mail: andrea.pochini@unipr.it
[
**] The Authors thank Mintek, South Africa (Project AuTEK), the BBSRC
(
Centre for BioArray Innovation) and the Italian FIRB (Manipola-
2
m sodium chloride solution. They exhibit a characteristic
zione Molecolare per Macchine Nanometriche) for financial
support.
plasmon absorption band in the UV/Vis spectrum at 526 nm
and their size is that of the original citrate-stabilized particles
(14 Æ 1 nm) as confirmed by TEM (Figure 2).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 2913 –2916
DOI: 10.1002/anie.200462909
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2913

Communications
spectrum of the particles shows significant peak broadening,
in particular of the methylene protons adjacent or close to the
sulfur atom that binds to the surface of the particle. This effect
is chiefly due to the lack of mobility of the closely packed
molecules in the ligand shell, which on the nanometer scale
resembles a solid-state material. Several other factors con-
tributing to line broadening in NMR spectra of MPCs have
also been suggested, and the phenomenon is still subject to
[
5,6,14]
some debate.
Although the broadening of the features
Figure 2. TEM image of the 14-nm gold nanoparticles 3, that are
stabilized and functionalized with 1 and 2.
impedes a more detailed interpretation, it is apparent that all
the main characteristic features of the two different ligands
are present in the spectrum of the particles. Importantly,
slightly broadened signals between d = 6.6 and 7.2 ppm, which
are characteristic of the aromatic protons in the calixarene
cavity are clearly present in the spectrum of
The molecular composition of the ligand shell has been
1
studied by H NMR spectroscopy (Figure 3). As expected, the
the particles. This result indicates the inclu-
sion of 2 in the ligand shell. It can be inferred
from previous studies of peptide-stabilized
MPCs of the same size that the number of
thiol ligands is typically between 800 and
[
18]
1
000 molecules per gold particle. Under the
assumption of similar adsorption kinetics for
both 1 and 2 it can thus be estimated that each
gold particle carries on the order of 300 mol-
ecules of 2. A quantitative analysis of the
binding properties of the material will be
required to confirm this estimate. Such experi-
ments are underway. The binding properties
of a solution-free, water-soluble model cal-
ix[4]arene derivative towards N-alkyl pyridi-
nium salts in D O are given quantitatively in
2
the Supporting Information. The molecular
recognition properties of the particle-bound
calixarene in water are qualitatively demon-
strated by specific binding studies using two
different chemically modified substrates
1
Figure 3. H NMR spectra of 2 (a) and 1 (b) in CDCl , and of 3 (c) in D O showing that 1 and 2
3
2
have been attached to the gold nanoparticles (the signals of the methylene protons adjacent to the
thiol and oligoethylene glycol (OEG) protons are marked). In particular, the signals of the aromatic
protons (Ar-H) in spectrum (c) indicate the presence of the calixarene cavity in the ligand shell of
the particles.
(Figure 4). In the simplest case the substrate
Figure 4. Specific binding of the calixarene-modified gold nanoparticles 3, from aqueous solution to a self-assembled monolayer (SAM) of the pyr-
idinium ions of 4 (Tos=(4-methylphenyl)sulfonyl) on a gold surface shown by AFM (a) and to a molecular sieve bead primed with 4 (b). The con-
trol experiments (right) show the nonspecific attachment of only very few particles to the SAMs (a) and no visible attachment of particles to the
molecular sieve beads (b).
2
914
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Angewandte
Chemie
was prepared by the nonspecific adsorption of 4 (Figure 4) to
white molecular sieve beads. For non-aqueous media it is
known that the cationic pyridinium moiety is specifically
4: Thioacetic acid S-[11-(toluene-4-sulfonyloxy)-undecyl] ester
a) was prepared by refluxing a mixture of toluene-4-sulfonic acid
(
undec-10-enyl ester (5 g, 15.4 mmol) and thioacetic acid (2.3 g,
31 mmol) in toluene (200 mL) for 3 h under exclusion of oxygen
and in the presence of a catalytic amount of 2,2’-azobis[(2-methyl)-
propanenitrile] (AIBN) and subsequent removal of the solvent by
rotary evaporation. The solid crude product was dissolved in CH Cl ,
[
20]
recognized by calix[4]arenes.
The nanoparticles 3 bind
specifically from aqueous solution to the molecular sieve
beads primed with 4 indicating that the calixarene cavity of 2
exhibits its characteristic cation binding properties also in
aqueous media and when immobilized in the ligand shell of
the MPCs. This binding interaction is easily observed with the
naked eye owing to the intense red color of the gold
nanoparticles. A number of control experiments clearly
confirmed that both the pyridinium and the calixarene
moieties have to be present to achieve any attachment of
MPCs to the molecular sieve beads. Further binding studies
were carried out by AFM using a self-assembled monolayer
2
2
washed with water and sodium hydrogen carbonate, and purified by
column chromatography (silica gel, hexane 90%/ ethyl acetate 10%)
to give a yellowish viscous solid (90% yield). Pyridine (0.5 g, 5 mmol)
was added to a solution of a in acetonitrile (2 g, 5 mmol, 100 mL) and
heated under reflux for 24 h. The solvent was removed by rotary
evaporation and the pure white solid product was obtained by
precipitation in ethyl acetate/acetone in good yield (80%). This white
solid (2 g, 4 mmol) and para-toluene sulfonic acid (1 g, 7 mmol) were
dissolved in methanol (20 mL) and heated under reflux overnight
then the solvent was removed by rotary evaporation. Precipitation of
the resulting mixture in CH Cl yielded a pure white sticky solid
2
2
(
SAM) of 4 on a flat gold surface as the substrate. As
product
4 (75% yield). SAMs of 4 on gold (gold films of
demonstrated in the AFM images in Figure 4a the calixarene-
modified nanoparticles 3 were found to bind selectively to this
SAM from aqueous solution, while binding to clean gold
surfaces occurred only as a non-specific minority event. MPCs
without calixarene in their ligand shell did not show any
significant binding to the SAMs or to clean gold surfaces.
In conclusion, we have introduced a very simple route for
the preparation of water-soluble calixarene-functionalized
MPCs of gold. Importantly, it has been demonstrated that in
an aqueous environment the calixarene retains its molecular
recognition properties, which have been utilized herein to
bind the MPCs selectively to chemically modified substrates.
The MPCs also act as readily detectable markers of the
specific recognition events demonstrating a high potential for
simple, color-based diagnostic tests, for example, those
required for many routine bioanalytical applications. The
preparative method is very general and can readily be
adapted to other artificial molecular recognition systems.
Particularly attractive is the opportunity demonstrated, to
make totally water insoluble recognition systems amenable to
aqueous solutions, which again is of interest in the context of
bioanalytical applications of MPCs.
approximately 250-nm thickness on quartz glass, Arrandee) were
prepared by overnight immersion of the cleaned and flame-annealed
substrates in a solution of 4 in CHCl3 (5 mL, 1.2 ꢀ 10 m) and
À4
subsequent thorough rinsing with CHCl and drying in a stream of
3
nitrogen. Single molecular-sieve beads (4 ꢁ type, Aldrich) were
modified by overnight immersion in solutions of 4 (2.5 mL, 1.2 ꢀ
À4
10
m) in chloroform. After removal from the solution, the beads
were washed thoroughly with chloroform and allowed to dry at room
temperature. Specific recognition experiments were carried out by
overnight immersion of the substrates (SAMs and beads) in a solution
of 3 (0.034 nm, 2.5 mL for SAMs and 1.2 nm, 1.5 mL for beads).
Adequate control experiments were performed to exclude the
possibility of nonspecific binding.
For spectroscopic data and elemental analyses see the Supporting
Information.
Received: December 11, 2004
Published online: April 7, 2005
Keywords: calixarenes · gold · molecular recognition ·
.
nanoparticles · water solubility
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2915

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